from the destination PE router. When the destination PE router receives the labeled packet, it pops the label
and uses it to direct the packet to the correct CE router. Label forwarding across the provider backbone is
based on dynamic label switching. A customer data packet carries two levels of labels when traversing the
backbone:
• The top label directs the packet to the correct PE router.
• The second label indicates how that PE router should forward the packet to the CE router.
Automatic Route Distinguisher Assignment
To take advantage of iBGP load balancing, every network VRF must be assigned a unique route distinguisher.
VRF is require a route distinguisher for BGP to distinguish between potentially identical prefixes received
from different VPNs.
With thousands of routers in a network each supporting multiple VRFs, configuration and management of
route distinguishers across the network can present a problem. Cisco IOS XR software simplifies this process
by assigning unique route distinguisher to VRFs using the rd auto command.
To assign a unique route distinguisher for each router, you must ensure that each router has a unique BGP
router-id. If so, the rd auto command assigns a Type 1 route distinguisher to the VRF using the following
format: ip-address:number. The IP address is specified by the BGP router-id statement and the number (which
is derived as an unused index in the 0 to 65535 range) is unique across theVRFs.
Finally, route distinguisher values are checkpointed so that route distinguisher assignment to VRF is persistent
across failover or process restart. If an route distinguisher is explicitely configured for a VRF, this value is
not overridden by the autoroute distinguisher.
L3VPN Configuration Guide for Cisco NCS 540 Series Routers, IOS XR Release 6.3.x
54
MPLS L3VPN Overview
Automatic Route Distinguisher Assignment
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